It has long been recognized in the coin examining art that the interaction of an object with a low frequency electromagnetic field can be used to indicate, at least in part, the material composition of the object and thus whether or not the object is an acceptable coin and its denomination. See, for example, U.S. Pat. No. 3,059,749. It has also been recognized that such low frequency tests are advantageously combined with one or more tests at a higher frequency. See, for example, U.S. Pat. No. 3,870,137. The optimum methods for low frequency testing have, in the past, used bridge circuits which incorporate testing of both phase and amplitude effects of coin interaction with an electromagnetic field.
Another technique which has been popular in the testing of coins has been the transmit-receive technique in which an electromagnetic field is created by an inductor adjacent one face of a coin and characteristics of the received signal adjacent the other face are examined to determine the coin's authenticity and denomination.
U.S. Pat. Nos. 3,599,771 and 3,741,363, for example, each discloses a transmitter coil creating an electronic field at either end. Spaced adjacent each end of the transmitter coil is a secondary coil. The two secondary coils are electrically connected in series, and have opposing orientations with respect to the transmitting coil field. An unknown coin is placed between one secondary coil and the transmitting coil and a known coin is placed between the other secondary coil and the transmitting coil. The unknown coin is accepted only if the signal delivered by the secondary coils does not exceed a threshhold value. Such an arrangement, of course, is suitable only for examination of one coin denomination per testing station.
U.S. Pat. No. 3,966,034, assigned to the assignee of the present application, discloses a phase sensitive coin discrimination method and apparatus operating by the transmit-receive technique with particular utility in distinguishing between two similar coins such as the British 5P and the West German 1DM. Unlike the present invention, the detailed embodiments of that patent operate at relatively high frequencies (e.g. 320 kHz) and rely upon differences in coin volume to help distinguish between otherwise similar coins.
U.S. Pat. No. 4,086,527, discloses a transmit-receive type coin examining apparatus in which the transmitter coil is driven by a controlled variable frequency oscillator operated at one or more selected frequencies in the range of 5-300 kHz. The secondary or receiving coil is connected to an undisclosed "quantifying operator" circuit which obtains quantitative information regarding amplitude of the secondary signal and its phase with respect to the primary (transmitted) signal.